1. Technical Field
This disclosure generally relates to tuning the working function of transparent electrode in electroluminescence (EL) devices, in particular, organic light-emitting diodes (OLEDs).
2. Description of the Related Art
An OLED emits light in response to an electric current. FIG. 1 shows a typical OLED (10) formed on a transparent substrate (20). An anode (30) is disposed on the transparent substrate (20) and is also transparent to allow the internally generated light to exit. The light-emitting layer takes the form of an organic emissive stack (40), which is disposed between the anode (30) and a cathode (50). The organic emissive stack (40) includes a thin film of electroluminescent chemical compounds (60) flanked by two charge injection layers (70 and 80, one for electron injection and one for hole injection).
Although indium tin oxide (ITO) is commonly used as the transparent electrode in an OLED, metal nanostructure-based transparent conductors represent an emerging class of transparent electrodes. Unlike the ITO, which is vacuum deposited on a substrate, metal nanostructure-based transparent conductors are formed by coating an ink formulation of metal nanowires on a substrate. The process addresses certain production limitations encountered by the ITO, and is particularly suitable for printing or coating on large area and/or flexible substrates.
The light generation mechanism of the OLED is based on radiative recombination of excitons of electrically excited organic compound(s). As a current of electrons flows through the OLED from the cathode to the anode, electrons are injected into the lowest unoccupied molecular orbital (LUMO) of the organic compound at the cathode and withdrawn from the highest occupied molecular orbital (HOMO) at the anode. The process of withdrawing the electrons from the HOMO may also be described as injecting holes into the HOMO. Electrostatic forces bring the electrons and the holes toward each other and they recombine forming an exciton, an excided state of the electron bound to the hole. The excited state relaxes to the ground state of the electron, accompanied by emission of radiation, the frequency of which is in the visible region (380-800 nm). The frequency of the radiation depends on the difference in energy between the HOMO and LUMO.
In addition to determining the frequencies of the emitted light, the energy levels HOMO and LUMO, as well as those of the electrodes, have significant impact on the efficiency and the performance of the OLED. FIG. 2 shows schematically an energy diagram of an OLED. The energy difference between the anode and HOMO represents an energy barrier (Eh) for the hole injection. Similarly, the energy difference between the cathode and LUMO represents an energy barrier (Ee) for the electron injection.
Work function of an anode (or cathode) corresponds to the minimum amount of energy needed to remove an electron from the surface of the anode (or cathode). As shown in FIG. 2, increasing the work function of the anode (e.g., to the dashed line) decreases the energy barrier (Eh), thereby increasing the efficiency of the hole injection from the anode.
The work function of a surface is strongly affected by the condition of the surface. For example, the work function of ITO can be increased from 4.2 eV to 4.8 eV by oxygen plasma. See, e.g., Wu, C. C. et al. Appl. Phys. Lett. 70 (11):1348 (1997). Changing the work function of a material by absorption of a thin layer of a substance with an electrostatic dipole has also been reported. See, e.g., Gu, D. et al. J. Appl. Phys. 97:123710 (2005).
There remains a need to adjust the work function of the metal nanostructure-based transparent conductor in an OLED device.